1. Field of the Invention
The present invention relates to lighting devices and projection-type display apparatuses including a lighting device, and more particularly relates to a lighting device furnished with a plurality of light sources and capable of changing the lighting timing and the intensity of each light source individually, and to a projection-type display apparatus including the lighting device.
2. Description of the Background Art
Lighting devices including a light source are incorporated in, for example, projectors that concentrate light emitted from the light source onto a display element called a DMD (digital micromirror device) or onto a liquid crystal panel and thereby display a color image (color video image) on a screen. Most such lighting devices incorporated in projectors or the like have used discharge lamps with high brightness as their light sources. In recent years, however, many devices that use LEDs (light emitting diodes), laser diodes, organic electroluminescence, fluorescence emission, or the like, have been developed and proposed.
For example, a lighting device has been proposed that includes a solid-state light source and a circular plate-shaped phosphor wheel that is composed of a transparent base material and on which a phosphor layer is formed. The phosphor layer receives ultraviolet light emitted from the solid-state light source as excitation light, and converts the ultraviolet light to visible light.
The proposed lighting device can emit fluorescence in wavelength bands of red, green, and blue, by irradiating the phosphor layer formed on the surface of the wheel with the excitation light. However, the excitation light, which is irradiated onto phosphors of red, green, and blue, cannot be changed for each phosphor. Therefore, in the proposed device, most suitable excitation light cannot be selected for each phosphor, which causes a problem of insufficient brightness of the fluorescence.
In view of the above problem, a lighting device has been proposed that uses a combination of a phosphor wheel having a phosphor of high luminous efficiency, a plurality of light sources for exciting phosphors, and a monochromatic light source for emitting light in a wavelength band appropriate for a phosphor of relatively low luminous efficiency.
FIG. 10 is a diagram illustrating a phosphor wheel used for a conventional lighting device. As shown in FIG. 10, the surface of the phosphor wheel is divided into a plurality of segment areas 901, 902, and 903 onto which different phosphors have been respectively applied. The segment areas are formed on the surface of the phosphor wheel so as to be adjacent to each other. With this structure of the phosphor wheel, desired color light can be obtained by controlling lighting and extinction of the plurality of light sources, and controlling rotation of the phosphor wheel in accordance with the emission of light.
As described above, the proposed lighting device can produce light of desired color by controlling rotation of the phosphor wheel such that, at the moment when a light source emits light, a predetermined phosphor is located at a position illuminated with the emitted light. However, as described above, the phosphors are applied only to the segment areas divided on the surface of the wheel. Therefore, in order to obtain white light, for example, the following steps must be performed. First, blue light is emitted from a first light source and transmitted through a transmission section of the phosphor wheel, and meanwhile, the phosphor wheel is rotated to irradiate a first segment area with the blue light and thereby to obtain green fluorescence. Then, the first light source is turned off, and red light is emitted from a second light source, and the fluorescence and the light are combined. A problem in this case is that with the blue light and green light being obtained by means of the first light source, the intensities of the obtained color light are not sufficient. Further, the proposed lighting device cannot produce light of a preferred color at a desired timing, which is inefficient.